In charged particle beam therapy, an energetic charged particle beam is directed at a target zone of a subject. The primary mechanism for the interaction of such a beam comprising charged particles with matter is through the Coulomb force. The cross section for Coulomb collisions increases as the relative velocity of two particles decreases. As a charged particle beam travels through a subject, it loses energy more and more rapidly. The effect of this is that the majority of the energy of the particle beam is deposited near the end of the beam path. There is therefore a large peak of energy deposited at the end of the beam path which is called the Bragg peak.
For this reason, charged particle beam therapy allows very precise delivery of high dose to a tumor target while minimizing the total dose to the subject. However, even small movements of anatomical structures in the path of the beam can lead to significant deviations of the delivered dose from the original dose plan. Therefore, it is desirable to use real-time imaging to track the target and adapt the beam to the motion of organs and of the target.
For charged particle beam therapy, real-time MRI during the delivery of the beam has been unfeasible, because the strong magnetic fields associated with MRI will dramatically impact the path of the charged particles towards the target.
A static magnetic field is used by Magnetic Resonance Imaging (MRI) scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a subject. During an MRI scan, Radio Frequency (RF) pulses generated by a transmitter coil cause perturbations to the local magnetic field, and RF signals emitted by the nuclear spins are detected by a receiver coil. These RF signals are used to construct the MRI images. These coils can also be referred to as antennas. Further, the transmitter and receiver coils can also be integrated into a single transceiver coil that performs both functions. It is understood that the use of the term transceiver coil also refers to systems where separate transmitter and receiver coils are used.
U.S. Pat. No. 6,675,078 and corresponding European patent EP 1 121 957 A2 describes a therapeutic apparatus which combines proton beam therapy with MRI. MRI is used for targeting and gating the proton beam therapy.
PCT patent application WO 99/32189 relates to a combined MRI and radiotherapy system. The system described has a magnetic resonance imaging system, a gantry mounted set of coils for generating a magnetic field that rotates with a guiding system for a radiotherapy beam, and it uses MRI to detect the effect of the radiotherapy on an irradiated region.